As a fine grating for reflection prevention, a grating in which conical or pyramidal protrusions are arranged cyclically two-dimensionally has been known (for example, GRANN, E. B. et al “Optimal design for antireflective tapered two-dimensional subwavelength grating structures”, Optical Society of America, February 1995, Vol. 12, No. 2, pages 333-339). On the base plate of the grating, a period in the X direction and a period in the Y direction are equal to each other and this period is set smaller than wavelengths of visible lights. In this specification, such a grating is called two-dimensional grating or two-dimensional subwavelength grating. It has been known that a two-dimensional subwavelength grating having conical grating protrusion parts (projecting portions) has antireflective effect on a wide wavelength band like an element in which plural thin film layers changing continuously are stacked.
As a method for manufacturing such a two-dimensional subwavelength grating, there has been known a method in which a resist undergoes exposing by turning ON/OFF electron beam with an electron beam exposing unit based on a cyclic dot-like pattern data so as to form a resist pattern and a substrate undergoes etching using this resist pattern as a mask so as to form a two-dimensional subwavelength grating having a desired structure on the substrate (for example, Japanese Patent Application Laid-Open No. 2004-85831). Further, there has been known also a method in which after a dot array-like metal mask is formed on an optical device, reactive ion etching is performed in such a way that the metal mask decreases gradually and it finally vanishes so as to form a conical portions (for example, Japanese Patent Application Laid-Open No. 2001-272505).
However, the methods mentioned above are complicated in their processes and cost much.
On the other hand, there has been known a method in which a two-dimensional subwavelength grating is formed with a mold in order to provide antireflective material having a low reflectance at a low cost (for example, Japanese Patent Application Laid-Open No. 2004-287238).
However, even in the above-described method it is difficult to improve a percentage transfer when a two-dimensional subwavelength grating is produced by injection molding using a mold and therefore, it is difficult to obtain a two-dimensional subwavelength grating having a high aspect ratio. A percentage transfer mentioned here refers to a ratio of a depth of a concave part in a molded product to a height of the corresponding protrusion part of the mold or a ratio of a height of a protrusion part of the molded product to a depth of the corresponding concave part in the mold. Further, an aspect ratio refers to a ratio of a height of the protrusion part of the grating to a arrangement period of the grating.
The reason why it is difficult to improve a percentage transfer when manufacturing a two-dimensional subwavelength grating by injection molding using a mold will be described below.
FIG. 7 is a drawing showing a shape of a conventional mold for use in manufacturing a two-dimensional subwavelength grating by injection molding. In FIG. 7, a cavity bottom face of the mold is indicated with reference numeral 103. Reference numeral 109 indicates a concave part in the mold for forming a protrusion part of the two-dimensional subwavelength grating.
FIG. 8 is a drawing showing a shape of protrusion parts of the grating formed with the mold shown in FIG. 7. In FIG. 8, protrusion parts of the grating are indicated with reference numeral 201. The four drawings in FIG. 8 show a plan view, a sectional view taken along A-A′, a sectional view taken along B-B′ and a sectional view taken along C-C′, from top to bottom.
In the plan view the diagonally shaded circular areas indicate protrusion parts of the grating. Areas besides the diagonally shaded areas indicate a plane corresponding to the cavity bottom face of the mold.
In the sectional view taken along A-A′, the sectional view taken along B-B′ and sectional view taken along C-C′, the diagonally shaded areas indicate cross-sections of protrusion parts of the grating. Assuming that the directions of A-A′, B-B′ and C-C′ are X direction, the protrusion parts 201 of the grating are arranged at a certain period Λ in the X direction and the Y direction. Although sectional shapes are of parabola in the drawing, the sectional shapes may be triangular or of other shape.
FIG. 9 is a drawing showing a cross-section of the mold shown in FIG. 7. In FIG. 9, a cavity of the mold is indicated with reference numeral 101 and a pouring port for synthetic resin is indicated with reference numeral 105. Like in FIG. 7, the cavity bottom face of the mold is indicated with reference numeral 103 and concave parts of the mold are indicated with reference numeral 109. A period at which the concave parts 109 of the mold are arranged is Λ and a depth of the concave parts of the mold is d. Here, period Λ is for example, 350 nm and depth d is 350 nm.
In FIG. 9, synthetic resin poured into the cavity 101 of the mold from the pouring port 105 flows along the cavity bottom face 103 as indicated with arrows, attempting to flow into the concave parts 109 of the mold. A diameter of the concave parts 109 of the mold on the cavity bottom face 103 is 350 nm which is equal to the period Λ.
Generally, assuming that a size of an object in fluid is L, Reynolds number which is a ratio of inertial force to viscous force is proportional to square of L. Thus, as the size decreases, the viscous force becomes dominant. Because the diameter of the concave parts 109 of the mold on the cavity bottom face 103 is 350 nm, which is very small, the viscous force is large with respect to a pressure, so that synthetic resin cannot easily flow into the concave parts 109 of the mold. As the cavity 101 of the mold is filled with synthetic resin so that the pressure of the synthetic resin increases, the synthetic resin gradually flows into the concave parts 109 of the mold.
The concave parts 109 of the conventional mold do not communicate with each other. Thus, air in the concave parts 109 of the conventional mold collects at the bottom portions of the concave parts 109 of the mold because it has no escape route. Consequently, synthetic resin more than a predetermined ratio cannot flow into the concave parts 109 of the mold due to pressure of the air. When a depth d of the concave parts 109 of the mold is 350 nm, a height of protrusion parts 201 of the grating formed with the concave parts 109 of the mold is as small as 180 nm. That is, the percentage transfer is only about 51%.
Thus, when a two-dimensional subwavelength grating is manufactured by injection molding using a mold, it is difficult to improve the percentage transfer due to viscous force and air collecting in the concave parts of the mold.
When a height of protrusion parts of the grating is 180 nm in the two-dimensional subwavelength grating manufactured with the conventional mold, reflectances of lights having a wavelength of 400 nm, 600 nm and 800 nm are 0.41%, 0.55% and 1.21% respectively.